In a network of a time division multiplex (TDM) system, a plurality of transmission devices reproduces clocks from data signals which are transmitted continuously, and thereby performs mutual time synchronization or frequency synchronization. The standard specifications of synchronous transmission by a time division multiplex system include, for example, synchronous digital hierarchy (SDH) and synchronous optical network (SONET).
Along with the proliferation of the Internet, Internet Protocol (IP) compatible networks have spread and are replacing time division multiplex systems. In contrast to the time division multiplex systems, data signals in IP are transmitted packet by packet asynchronously, and thus time synchronization or frequency synchronization is performed by different systems.
The standard specifications of time synchronization or frequency synchronization system based on IP include network time protocol (NTP), IEEE1588-2008 (IEEE stands for the Institute of Electrical and Electronics Engineers, Inc.). The technology specified in the IEEE1588-2008 is supposed to enable time synchronization with an accuracy of microseconds and is expected to be applied to processing such as synchronous processing between base stations in a radio network.
In the case of IP, synchronous processing is performed in such a manner that a synchronous message including a time stamp (time information) is exchanged between a server system and a plurality of client devices, and thereby a transmission delay time therebetween is measured and offset amounts of frequency and time are estimated. The synchronous message is exchanged via packet relay devices (for example, a layer 2 switch and a router) which are provided in a network. In the present description, a packet including a synchronous message is simply denoted as a “synchronous message”.
The packet relay devices relays not only a synchronous message but also a packet in other traffic similarly. For this reason, in the transmission processing of the packet relay devices, a conflict may occur between a synchronous message and another packet which is transmitted from the same port. In the case where a conflict occurs, even when a synchronous message is selected as a transmission candidate having a higher priority, if transmission processing (read from a queue) of another packet in conflict is already started, the synchronous message is kept on standby until the transmission processing is completed.
Therefore, a transmission delay time of a synchronous message in a network varies due to an effect from other traffic. This delay variation is called a packet delay variation (PDV) and could be a factor for reducing the accuracy of time synchronization and frequency synchronization.
On the other hand, for example, Japanese Laid-Open Patent Publication No. 2008-22131 discloses a technology that adjusts a delay in such a manner that a priority packet is stored in a priority queue not for a non-priority packet, and the priority queue is given a fixed delay time which is longer than or equal to the read processing time for a user packet with a maximum length. In addition, Japanese Laid-Open Patent Publication No. 2012-34224 discloses a technology that adjusts a delay in such a manner that a time synchronous packet is stored with a priority over other data packets and is transmitted with a delay time which is longer than or equal to the transmission time for a user packet with a maximum length.